The long-term objective of this program is to understand the mechanism(s) of cellular changes fundamental to neoplastic transformation. As nonneoplastic mammalian cells have specific hormone and growth factor requirements for initiation of DNA synthesis and mitosis, and retroviral oncogene (v-onc) gene product(s) can subvert normal growth regulatory mechanisms and cause neoplastic transformation in culture, the interaction(s) between v-onc gene products and subversion of signal transduction in the normal mitotic pathways or prototype epithelial cells was assessed in a serum-free bioassay system(s). Infection of BALB/MK keratinocytes with retroviruses containing v-ras oncogenes (v-H- ras, v-Ki-ras) with oncogenes which encode growth factor receptors (v-erbB, v-fms) or with v-mos permitted growth in defined medium containing insulin but lacking EGF. The v-fcr oncogene was unique as it conferred independence from both exogenous insulin and EGF. No evidence of a novel, secreted mitogen was detected in conditioned medium (CM) generated by monolayers of the fgr transformant. To determine if low passage, morphologically normal mesenchymal cells derived from human tumor cells secrete undefined mitogens, CM generated by monolayers from several organs was assayed. Modest mitogenic activity (greater than fivefold background for BALB/MK keratinocytes) was observed with CM from colon and stomach fibroblasts. If the incidence of intermediate activity (two- to fivefold background for BALB/MK cells) is included, CM from lung fibroblasts also contained significant undefined mitogenic activity. To date, the additional epithelial strains derived from tissues functionally or anatomically more similar to the tumor tissue than keratinocytes have not demonstrated a marked tissue specificity.